1. Field of the Invention
The present invention relates to a drive circuit for a magnetic cassette reader.
2. Discussion of the Related Art
Modern magnetic cassette readers normally feature an autoreverse function for automatically reversing the tape so that both sides of the cassette can be read without turning the tape over relative to the read head.
To perform the autoreverse function, the read head has two read sensors, or four for stereo, which are connected alternatively, depending on the drive direction of the tape, to a preamplifier stage by an autoreverse drive circuit by which the read sensor is selected as a function of the logic value of a digital input signal. On known readers, the drive circuit typically uses bipolar devices, which have several drawbacks due to their physical and electric characteristics.
Bipolar transistors introduce considerable switching noise which is further amplified by the preamplifier stages; are difficult to control; present a poorly controllable switch edge; and, in the case of integrated circuits, have a substantial bulk.
Modern cassette readers also have a mute function whereby the output of the preamplifier stage, and consequently also of the final stage, is attenuated considerably, typically by about 80 dB, regardless of the drive direction of the tape.
The mute function is typically affected by bipolar transistor switch circuits or attenuators, but these circuits also have several drawbacks. More specifically, in addition to the problems posed by the presence of bipolar transistors on the autoreverse drive circuits, bipolar transistors in mute circuits also fail to provide a high degree of attenuation.
Difficulties are also encountered in forming the autoreverse and mute function drive circuits on the same integrated circuit, as well as in simultaneously controlling the switching of the reverse and mute circuits.